The invention generally relates to a printing machine and more particularly relates to a sheet-fed rotary printing machine including at least one coating unit.
Printing machines are known which have multiple coating units. For example, patent publication EP 0 620 115 B1, relates to a rotary printing machine having an in-line process for coating sheet printing materials, including at least two varnishing units provided as coating units. One of the varnishing units, which is upstream relative to the sheet transport direction, is constructed as a flexographic printing unit. The flexographic printing unit has a plate cylinder that bears a relief printing plate and is in contact with the sheet-carrying printing cylinder, an engraved applicator roller that is in contact with the plate cylinder for the purpose of inking or coating. Additionally, the printing unit includes a chamber-type coating doctor which can be moved into contact, and which is connected to delivery and suction pumps for the supply and return flow of coating liquid from the chamber.
In conventional previous printing machines, it has been found that the coating fluid can become undesirably contaminated, resulting in poor printing or quality fluctuations. This is especially problematic in sheet printing machines with printing units for multicolor printing and at least one coating unit, for example, a flexographic printing unit and/or a varnishing unit. As a result of excess or dried coating fluid accumulated on the applicator rollers, the print quality undesirably fluctuates or is impaired as early as during the coating process.
Attempts have been made in conventional printing machines to reduce undesired coating fluid from the applicator roller. For example, German patent publication DE 195 26 574 C1 discloses a system for cleaning a plate cylinder and an applicator roller, whereby the drying of rapidly drying media onto the applicator roller and onto the plate cylinder is reduced. The washing apparatus has two curved surfaces which face the applicator roller and the plate cylinder.
Also, German patent publication DE 196 45 934 A1 relates to an engraved applicator roller having dimples which can be filled with a liquifiable substance as a gravure printing plate. By means of an ultrasonic cleaning system, the plate can be regenerated to a basic pattern of the dimples.
German patent publication DE 41 21 017 C2 relates to a printer cleaning apparatus having a soft, smooth rubber-coated roller and an associated stripper roller with a polygonal cross section. In order to remove printing ink from the cover of the stripper roller, an ultrasonic cleaning device is provided.
A chamber-type wiper metering system is generally disclosed by U.S. Pat. No. 5,121,689. In this case, the metering system includes a working wiper coupled to an ultrasonic device in order to meter the layer thickness of printing ink on the metering roll.
A need exists for an improved printing machine which provides improved print quality. A further need exists for an improved means of cleaning of coating fluid from the applicator rollers therein.
It is an object of the invention to provide an improved sheet-fed rotary printing machine. Another object of the invention is to provide stable, consistent print quality. Still another object of the invention is to reduce undesired build up of ink or coating fluid residues on rollers in a printing system.
The present invention achieves these objects and avoids the aforementioned disadvantages. Advantageously, a system according to the present invention provides stable quality during the coating process and noticeably reduces the contamination of coating fluid.
An advantage of the present invention is that it provides, in association with a metering system, a means for cleaning the applicator roller continuously or selectively, thereby reducing contamination. The invention may be used with a variety of structures. For example, the invention may be alternatively used with a smooth applicator roller instead of an engraved applicator roller (with dimples and webs).
A further advantage of the present invention is that it especially improves quality in systems having multiple printing units. In particular, a problem with conventional printing machines having multiple printing units, a splitting back of ink (flexographic and/or offset printing ink from the preceding printing process) from the printing material into the metering system would undesirably occur, particularly in the metering system of the first coating unit. This splitting-back of ink results from the printing process in the printing units arranged upstream in the conveying direction. For example, when a chamber-type wiper is used with an engraved applicator roller, residues of ink, varnish, etc. can be eliminated. Otherwise, such residues may accumulate in the interior of the chamber-type wiper and/or on the applicator roller, and thereby undesirably mix with coating fluid. The present invention avoids such an undesired splitting-back effect by cleaning the applicator roller.
Moreover, in printing systems wherein dual-roll units are used (based on the squeeze-roll principle, with a nip formed between the rolls), the present invention can be used to reduce the accumulation of ink residue in the nip formed by the metering roll and/or the applicator rollers. This likewise applies to metering systems based on the dip-roll principle. In this case, the contamination of the holding container, of the roll train and/or of the applicator roller can be avoided.
A further advantage of the present invention is to provide a cleaning system which reduces the number of cleaning intervals for the respective metering system with applicator roller.
Additional features and advantages of the present invention are described in, and will be apparent from, the description figures and claims herein.